The present invention relates to an electric discharge machine for machining a workpiece by generating electric discharge in a working gap formed between a tool electrode and the workpiece. More particularly, the invention particularly relates to an electric discharge machine power supply and method for repeatedly applying current pulses to the working gap using a capacitor.
When power is applied from a d.c. power source to the working gap, which is a microscopic gap formed between a tool electrode of an electric discharge machine and a conductive workpiece, the resistance of dielectric fluid in the gap is reduced. As the insulation characteristics of the dielectric fluid break down, an electric discharge occurs and the so-called pulse ON-time commences. During the controlled ON-time, electric discharge machining current flows through the working gap. As a result, workpiece material is evaporated or melted. Upon completion of the ON time, application of power is suspended during a controlled OFF-time, during which the insulation properties of the dielectric fluid is restored. In this way, current pulses having a controlled ON-time and a controlled OFF-time are repeatedly supplied to the working gap. A steep rising edge of the current pulses waveform is known to contribute to improved machining speed.
FIG. 4 is a circuit diagram illustrating an electric discharge machine power supply according to the related art. A workpiece 2 is arranged in a work tank (not illustrated) filled with dielectric fluid such as kerosene. A tool electrode 1 is positioned so that a working gap 3 of microscopic size is formed between the tool electrode and the workpiece 2. N series combinations made up of switching transistors Tr1-Trn and current limiting resistors R1-Rn are connected in parallel between a d.c. power source E and the working gap 3. In order to simplify the drawing, only two series combinations are shown, the other series combinations have been omitted from the drawing for clarity. An on/off switching operation of the switching transistors Tr1 to Tr2 is controlled by a gate pulse signal GP.
Electronic components such as a unit for generating the gateway signal GP, a d.c. power supply E, switching transistors Tr1-Trn, and current limiting resistors R1-Rn are normally housed in a cabinet. This type of cabinet is invariably arranged at a physical distance from mechanical sections such as a member for supporting the workpiece and a member for providing relative allowing movement between the tool electrode and workpiece. In order to electrically connect the d.c. power source E and the working gap 3, a suitable conductor such as a coaxial cable CC is provided between the cabinet and the mechanical sections.
The power supply of FIG. 4 further includes a capacitor C connected in parallel across the working gap 3. A switch SW is connected between the capacitor C and the working gap 3, and it is possible to selectively use the capacitor C. The electrostatic capacity of the capacitor can be set in a range of, for example 0.0068-1.6 xcexcF. The combination of the capacitor C and the switch SW is arranged as close as possible to the working gap 3. The capacitor C and switch SW are housed, for example, in a small box attached to a side wall of the work tank. If at least one of the switching transistors Tr1-Trn is turned on with the switch SW in the closed state, the capacitor C starts to charge. When the charging voltage of the capacitor C exceeds a certain value, an electric discharge current I flows through the work gap 3. At the same time as current is supplied from the capacitor C to the work gap 3, current is also supplied from the d.c. source E through the switching transistor to the working gap 3. A current pulse waveform supplied from the capacitor C to the working gap 3 is characterized by a steep rising edge. This steep rising edge improves the machining rate. The circuit of FIG. 4 is particularly useful in cases such as where a copper electrode is used so that the surface of a steel workpiece may be finished with a surface roughness of 3 xcexcmRy or less, or where a copper tungsten electrode is used when machining a sintered hard metal workpiece.
The time taken for an electric discharge to commence after the switching transistors are turned on varies depending on the condition of the working gap 3. As a result, there is the drawback that current supplied from the capacitor C to the working gap is not constant. For example, an electric discharge may commence in the working gap before there has been sufficient charging of the capacitator. Also, even if the switching transistors Tr1-Trn are off, it is possible for an unexpected discharge to occur in the working gap 3 because of electric charge stored in the capacitor C.
An object of the present invention is to provide an electric discharge machine power supply that can control a single charge of current pulse energy supplied from a capacitor to a working gap so as to be essentially constant.
Another object of the present invention is to provide an electric discharge machine power supply and method that can sufficiently ensure sufficient time to enable capacitor charge to be stored. Additional objects of the invention will be set forth in the description which follows, and will in part become apparent to those skilled in the art upon practicing the invention.
In order to achieve the above and other objects, one aspect of present invention is directed to an electric discharge machine power supply and method which repeatedly provides a current pulse to a working gap formed between a tool electrode and a workpiece, the power supply comprising a d.c. power source, first switching elements connected between the d.c. power source and the working gap, a capacitor connected in parallel with the working gap, second switching elements for controlling current flow from the capacitor to the working gap, a detector for detecting the start of an electric discharge, and a controller for controlling the first switching elements and the second switching elements in response to the detector so that current is supplied from the d.c. power source through the first switching elements to the working gap only for a first time interval from commencement of electric discharge, and current is supplied from the capacitor through the second switching elements to the working gap for only a second time interval, shorter than the first time interval, starting from commencement of an electric discharge.
According to another aspect of the present invention, there is provided an electric discharge machining power supply by repeatedly providing a current pulse to a work gap formed between a tool electrode and a workpiece; the power supply, comprising a d.c. power source, first switching elements connected between the d.c. power source and the working gap, a first cable for directing current from the d.c. power source through the first switching elements to the working gap, a capacitor connected in parallel with the working gap, a second cable for directing charging current from the d.c. power source to the capacitor, second switching elements for controlling charge current flowing from the capacitor to the working gap, and a controller for controlling the first switching elements and the second switching elements.